Due to high efficiency and precise control abilities, permanent magnet (PM) rotating machines are commonly used as control motors, typically servo motors. In AC servo motors, for example, a permanent magnet rotating machine with a radial air gap as illustrated in FIG. 7 is used. This PM rotating machine comprises a rotor 3 including a rotor core 1 and a plurality of D-shaped permanent magnet segments 2 attached to the surface of the core, and a stator 13 surrounding the rotor 3 to define a gap therebetween and including a stator core 11 having a plurality of slots and coils 12 wound on teeth and received in the slots. In the PM rotating machine illustrated in FIG. 7, the number of (permanent magnet) poles is six (6), the number of teeth is nine (9), and the arrow associated with a permanent magnet segment indicates a direction of magnetization thereof. With regard to the permanent magnet segments, magnetic orientation is effected in a parallel magnetic field so that a direction of easy magnetization is parallel to the center axis of the magnet segment. The coils are wound on teeth as a concentrated winding and connected in Y connection of three phases: U, V and W phases as shown in FIG. 8. In FIG. 7, the solid circle of a coil denotes that the coil winding direction is forward and the crossing (X) of a coil denotes that the coil winding direction is backward, with respect to the plane of paper.
When electric current flows across coils, magnetic fields are developed in the directions of broad arrows depicted in the stator core region, so that the rotor is rotated counterclockwise. At this point, an aft area of a permanent magnet segment in the rotating direction (a circled area of segment 2 in FIG. 7) is in a situation susceptible to demagnetization because the magnetic field is in an opposite direction to the magnetization of the permanent magnet segment. Commonly used permanent magnet materials include ferrite magnets such as barium ferrite and strontium ferrite and rare earth magnets such Sm—Co and Nd—Fe—B magnets. Of these, the rare earth magnets now find a dramatically increasing use as the high-performance magnet.
In AC servo motors and similar motors requiring high precision torque control, the torque must have less ripples. Accordingly, it is undesired that when the permanent magnets rotate, the alignment of stator slots and the permanent magnets causes cogging torque to develop due to variations of the magnetic flux distribution across the gap (i.e., torque without current flowing across the coil) or torque ripples to occur when driven by current flowing across the coil. The torque ripples exacerbate controllability and additionally, cause noise.
The cogging torque may be reduced by configuring a permanent magnet segment to a cross-sectional D shape that tapers from the center toward transverse ends and includes an off-centered arcuate portion as shown in FIG. 9. With this configuration, the end portion of a permanent magnet segment which is a magnetic pole switch area developing a noticeable variation of magnetic flux distribution produces a smoothened magnetic flux distribution, reducing the cogging torque. The cogging torque may also be reduced by configuring a permanent magnet segment to a cross-sectional C shape that includes an off-centered arcuate portion as shown in FIG. 10. The D-shaped magnet is more effective for reducing cogging torque even if slightly off-centered, because its transversely opposed end portions are thinner than its central portion. Although off-centered magnets have a reduced volume and thus lead to a reduction of drive torque, a proportion of torque reduction is smaller with the magnet of D shape which can reduce cogging torque with a less off-centering. Then the magnet of D shape is superior to C shape.
An off-centered D-shaped magnet as shown in FIG. 9 has end portions which are very thin and susceptible to demagnetization. Now the reason why a thin gage magnet end portion is susceptible to demagnetization is described. The magnitude of demagnetization of a permanent magnet is determined by the magnitude of a coercive force and the magnitude of a diamagnetic field at the service temperature. The demagnetization susceptibility increases as the coercive force is lower and as the diamagnetic field is greater. The diamagnetic field is the sum of a self diamagnetic field created by magnetization of a permanent magnet and a reverse magnetic field from the exterior. To the permanent magnet end portion is applied a high diamagnetic field from the stator. The self diamagnetic field increases as the thickness of the permanent magnet in the magnetization direction is reduced.
Once the magnet is demagnetized, there arise problems that the drive torque is reduced and the cogging torque is increased due to a locally uneven magnetic field.
Reference should be made to JP-A 2006-60920.